An apparatus of the kind given above is described in Proc. 34th Ann. Freq. Control Symposium, USAERDCOM, Ft. Monmouth, N.J. 07703, May 1980, pp. 202-212 (see particularly FIG. 10 with pertaining text).
Such an apparatus is also shown in the appended FIG. 1. A voltage controlled oscillator is denoted by the numeral 1, and its frequency is roughly set by a voltage source 2 which, in response to the selected frequency of the oscillator, generates one of a plurality of different voltages. The frequency is finely adjusted by a correction voltage formed by the oscillator output signal being phase locked to the signal from a reference oscillator 3. The latter being connected to a phase angle-sensing means or phase angle sensor 4 disposed to send on its output a control signal in the form of a short pulse, each time the phase angle of a signal at its input is equal to a given value, e.g. zero. A comparison means or comparator 5 in the form of an interacting and holding circuit is activated when the short pulse occurs. The means 5 then senses the output signal of the voltage controlled oscillator and conventionally generates a correction voltage which, via a low pass amplifier means 6 containing an amplifier and a lowpass filter is supplied to the voltage controlled oscillator 1 for correcting its frequency.
An example of the signals' variations in time at the points a-c is illustrated by the diagram of FIG. 2. The graphs 2a, 2b, and 2c respectively represent the signal from the reference oscillator 3, the short pulses from the phase angle sensing means 4, and the signal from the voltage controlled oscillator 1. In the example the frequency of the controlled oscillator is 4 times as great as that of the reference oscillator.
In the illustrated apparatus of the prior art, only signals having frequencies constituting whole number or integer multiples of the reference oscillator frequency may be phase locked to the signal from it. The frequency spacing between frequencies which may be phase locked is thus equal to the reference oscillator frequency. However, in a radar system, for example, there is often required a larger number of frequencies in a given frequency band than can be obtained from this apparatus. The number of phase lockable frequencies can indeed be increased if the reference oscillator frequency is reduced. This means accordingly that the sampling rate is decreased and that the amount of multiplication must be increased. These changes result in that the generated signal is deteriorated due to socalled phase noise. Another conceivable possibility of increasing the number of available frequencies is to change between different mutually frequency shifted reference oscillators on changing frequency. This causes leakage problems, however, between different oscillator signals as well as high costs for oscillators and switches.